1. Field of the Invention
The present invention relates to an optical scanning system used for a laser printer or a similar optical device. More precisely, it relates to an optical scanner having a high resolution and a small f-number, in which little or no displacement of focal point occurs due to a change in temperature.
2. Description of Related Art
To reduce the manufacturing cost or weight of a scanning lens system in a laser printer, the lens, or lenses, is usually made of plastic. However, the linear expansion and refractive index of plastic is more significantly influenced by the change in temperature than that of glass, resulting in the displacement of the focal point. This results in a deteriorated quality of print.
To eliminate the adverse effect caused by the temperature variation, it is known to provide a plurality of plastic lenses in combination within the scanning lens system so that the displacement of the focal point caused by the temperature change is cancelled.
However, in the above-mentioned known solution in which temperature compensation is carried out by a plurality of plastic lenses in combination, the number of lenses in which the scanning lens system is comprised is increased. This prevents the realization of a compact optical scanner and the reduction of the manufacturing cost.